This invention relates to polymeric packaging film, more particularly to consumer plastic wrap made of polyolefins. Such films have cling wrap properties which are useful for households and shops such as for wrapping and display of foods in refrigerators.
Various materials and processes have been used in the manufacture of such films, the most common materials being polyvinylchloride (PVC) polyvinylidene chloride (PVDC) copolymer and polyolefins, such as polyethylene. However, conventionally made films suffer from various disadvantages and have not been completely satisfactory in use.
Polyvinylchloride film must be highly plasticized to possess high strength characteristics such as tensile strength, or puncture resistance. This high level of plasticizer can give a "chemical" flavor to food especially when used at elevated temperatures.
Polyethylene films made of a blend of high density polyethylene (HDPE) and low density polyethylene (LDPE) have good handling, as measured by two percent secant modulus. Such films, however, have low strength, which is measured by a puncture, propagation and tear (PPT) test. The low PPT value results in films that tend to tear in strips during dispensing and handling, which is problematic for consumer wraps.
Films made completely of LDPE usually have good optical properties and good tear resistance. However, such films have a low two percent secant modulus, i.e., below 25,000 psi. Low modulus films tend to be limp and hard to handle. The cling property in this type of film is typically enhanced by the use of a cling additive. These films are not preferred in market-research studies as good consumer "high-cling" films. Cling is defined as how well the films stay wrapped around foods such as sandwiches, and adhere to open containers such as glass bowls. Low modulus films often tangle and fold over on themselves which is exacerbated when a cling additive is in the films.
Consumers are also interested in the ease of dispensing and tearing off a sheet of film as it is withdrawn from the container in which the film is packaged. For this purpose, containers such as cartons are ordinarily provided with sharp edges, usually a saw-toothed edge known as a cutter bar. Manufacturers have attempted to comply with this consumer demand by supplying film that can be withdrawn from a container to a desired length and severed along the cutter bar with ease. Even at 0.5 mils thickness, low density polyethylene film and especially linear low density polyethylene film (LLDPE) is very difficult, and in some instances, impossible to tear off in the transverse direction of the film on conventional cutter bar strips. The film tends to elongate and considerable effort and energy are required to sever the film.
Accordingly, it is an object of this invention to provide a transparent plastic film of low density polyethylene which has high strength characteristics, good handling and easy cutter bar tear.
It is another object of this invention to provide a process for the production of films of olefinic resins having high strength characteristics, good handling and easy cutter bar tear.